Remove as much Haskell code as we can from Lexer.x

I want to reduce the code in Lexer.x so (1) when I use hasktags I jump to some real source, not the postprocess output, and (2) so that I can avoid suppressing warnings on our Haskell source code, and not just the lexer code.

If we are willing to introduce an hs-boot file with the following signatures:

bol :: Int
layout :: Int
layout_do :: Int
layout_if :: Int
layout_left :: Int
option_prags :: Int

We can do quite well; the only functions we can't extract are: begin, pop, multiline_doc_comment, lineCommentToken, nested_doc_comment, withLexedDocType, do_bol, setLine, setFile, warnTab, addTabWarning, lexer, lexTokenAlr, lexToken, lexTokenStream, linePrags, fileHeaderPrags, ignoredPrags, oneWordPrags, twoWordPrags, dispatch_pragmas and known_pragma.

Some of these, like lexToken, we can't expect to be able to move to a separate file, as they depend on Alex generated data types and functions (AlexRoutine, alexScanUser). But if we boot-ify lexToken, we can move even more code out.

One question, however, is the performance cost of going through an hs-boot file, as these don't inline. I think the constants are fairly safe (inlining opportunities should only occur when we compile Lexer.x, at which point we will have unfoldings for them), but I am less certain about lexToken.

Any advice?

Trac metadata

Trac field	Value
Version	8.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Parser)
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